A Direct Injection Box (DI Box), also known as Direct Input Box, is a small piece of equipment that can be a life-saver when you are recording or even performing live. But what exactly does it do? Well, as it turns out, it does quite a lot.
A DI Box fulfills three basic roles:
- Impedance Matching: it converts High-Impedance Signals into Low-Impedance;
- Signal Balancing: it can be the first element of a Balanced Circuit;
- Gain Staging: it converts Instrument and Line levels in Microphone Level.
Still doesn’t make any sense?
Let’s dive in.
The Roles Of a DI Box
Impedance Matching
An audio signal is called analog when the electrical signal flowing in the wire correlates – is “analogue” – to the sound wave it represents. The stream of electrons (current) is not constant, but rather irregular, representing the peaks and valleys of the transduced sound wave.
That is what we call alternated current or AC.
Impedance (Z) is to alternated current (AC) what Resistance (r) is to direct current (DC). They are both measured in Ohms and ultimately represent the same concept: how much does a device resist the flow of electrical current.
So why do we need to convert impedance?
Because impedance mismatch can have a nasty effect on the sound with the introduction of noise and undesired distortion.
Impedance mismatch happens every time you connect a high impedance source into a low impedance input – or – a low impedance source into a high impedance input.
Here is an illustrative example for matching impedance with a DI box:
High Impedance Sources: Guitars, Basses, old ‘amp’ keyboards
Low Impedance Inputs: Mic Pre-Amp
The solution for matching the impedance is to connect the guitar, bass, or old keyboard (high impedance sources) into a DI Box or any impedance matching circuit, and then connect the DI output (low impedance source) into the microphone pre-amp (low impedance input).
Note that some Mic Pre-Amps do have a Hi-Z input, usually located at their front rear. Some Audio Interfaces also have pre-amplified dedicated instrument inputs. These are high impedance inputs designed to receive signals from high impedance sources. It is completely fine to connect a guitar or a bass directly into a Hi-Z or Instrument inputs without using a DI as a mediator.
Signal Balancing
A cable is nothing but a long piece of wire(s) through which electric current flows. Do you know what else is also a long piece of wire with flowing electric current?
Antennas.
And cables, like antennas, end up picking up all kinds of noise from outside sources. The longer the cable, the worse the results.
Most potential sources of noise are related to Electro-Magnetic Interference (EMI) such as TV and Radio Broadcast, Power Lines, Transformers, DC Motors, Lighting Equipment, Mobile Networks, Wireless and Bluetooth devices, all sorts of electronic equipment, and even Lightning and Solar Flares.
A balancing circuit eliminates these outside interferences (a.k.a Common Mode Rejection), allowing the signal to travel safely – and quietly – over long distances.
For a balanced system to exist, 3 conditions need to be met:
- Sender: the equipment sending the signal must have a balancing circuit that will output two versions of the signal: the original and a copy with inverted polarity – 180deg phase-shift. As the two signals travel through a balanced cable, they will be equally subjected to the introduction of noise.
- Receiver: the equipment receiving the signal must also have a balancing circuit that will restore the signal. Upon arrival, the signal with inverted polarity will have its polarity flipped on more time (being back in phase with the original) and the two signals will be summed. The original signal remains strong and intact, while the noise – that was equally introduced in both wires and is now out-of-phase by 180deg – will be completely canceled out.
- Path: Both ends need to be connected by a balanced cable. A balanced cable has either an XLR or a TRS connector. Running inside the outer insulation layer (cable jacket), you will find two conducting wires surrounded by a copper mesh:
- Red wire: Positive (or Hot).
It carries the original signal and should be connected to the XLR Pin 2 or TRS Tip. - White wire: Negative (or Cold).
It carries the signal with inverted polarity and should be connected to the XLR Pin 3 or TRS Ring. - Copper Mesh: Shield (or Ground).
It should be braided and connected to the XLR Pin 1 or TRS Sleeve.
- Red wire: Positive (or Hot).
Instruments do not have built-in balancing circuits, so they will always output unbalanced signals. DI boxes do have balancing circuits, converting an unbalanced signal into balanced from that point on in the signal path.
Gain Staging
Different instruments and equipment output different signal levels. Your problems begin when you try to feed a piece of equipment with a signal level that is different from what it is expecting to receive. That is called gain staging mismatch.
Elec Guitar, Basses, Mandolins, Violins, etc – all output Instrument Level
Keyboards, Laptops, CD Players – output Line Level (they all have Line Out)
Microphones – output Mic Level.
In the name of science, let’s conduct a weird thought experiment:
What would happen if you got your hand into a hybrid connector cable (¼ in TRS to XLR Male) and tried to connect your Bass directly into a pre-amp mic without a DI box?
First, the obvious impedance mismatch. Nevermind the fact that the Bass does not output a balanced signal, making the whole system unbalanced. To top it all, we would still have to deal with the gain staging mismatch:
The Bass outputs Instrument Level, which usually ranges around -20dBu. On the other hand, the Mic Pre Amp is expecting to receive Mic Level, on average -30dBu. In order for this combo to work, you would need to bring down the signal level from Instrument to Microphone.
If we replace our Bass with a Keyboard, that outputs Line Level (-10dBu), our results would be even worse.
Simply put, a DI box will always output Mic Level, guaranteeing signal levels will be matched.
Types Of DI Boxes
DI Boxes come in various shapes and forms. Which option is the right one for you?
It depends on what are you going to use it for.
Active vs Passive
As a general rule, If a source does not have internal power (passive source), such as electric and acoustic guitars, electric basses and mandolines, it is recommended to use an Active DI. The Active DI circuit requires batteries or +48V (phantom) in order to power their gain amplifier, which makes them a very popular choice for driving weaker signals. My favorite one, the Radial J48 MK2 48V Active DI, has been in my collection for the past 5 years.
However, have in mind that Active DIs may color the sound, i.e. slightly change its tone by introducing a small amount of harmonic distortion.
If a source has some form of internal power (active source), such as a keyboard, a bass with active pickups, a laptop or a CD Player, it is capable of producing high enough output levels to overdrive an Active DI. In those cases, it is recommended to use a Passive DI. I love the Radial Pro DI Passive DI, and surely many people share my opinion as they are fairly common in studios all over the world.
Mono vs Stereo
Stereo DI can handle two channels in the same box. They are great for sources with two output channels (L/R), like laptops and keyboards. Whenever you are dealing with a single source with stereo outputs, you should give preference to using stereo DIs.
Why? Because you want to make sure your signal (that happens to be in 2 channels) undergoes the same circuit, minimizing potential differences in level or tone between the channels. Even two mono DI from the same make and model can have slight differences. It is the same concept as having a set of studio monitors that do not have consecutive serial numbers – or wearing socks of different colors. Simply not ideal.
You may notice that, more often than not, stereo sources are active (keyboards, laptops, CD players). My go-to for these situations is the stereo version of the Radial Pro, the Radial ProD2 Passive 2-Channel DI.
How To Connect a DI Box
In every DI Box, you will find three connections:
Input: it comprises a ¼ inch Jack unbalanced connector (TS) where you will plug in your source (guitar, bass, keyboard).
Output: it consists of an XLR balanced connector where you will plug the equipment that will receive the balanced signal from the DI (mic pre-amp).
Thru: this is where things get interesting.
In a recording studio situation, It is extremely common to record electric guitars by mic’ing the guitar amp (Hi-Z input) and also by capturing the direct sound into a mic preamp (Lo-Z input) as a backup. You should know by now you need to place a DI between your guitar and the mic pre-amp.
If only there was a way to send a copy of the original signal from the guitar straight into the guitar amp before its impedance gets converted by the DI…
Well, that is exactly the point of the Thru connection.
The DI Box splits the signal in two just before it goes into its circuit. Part of the signal leaves the DI the same way it came in (without any processing), via the Thru connection (¼ inch Jack unbalanced connector (TS), the same as the input connector). The other part travels through the balancing circuit, has its impedance converted, and comes out the regular Output connection.
Common DI Box Settings and Controls
Pad or Attenuation: If your signal is arriving too hot (loud and distorted) at the other end, you can attenuate it by a fixed amount (usually somewhere between 15 and 20 dB) with the click of a button.
Polarity: reverse the original signal polarity, shifting the phase by 180deg
Merge: The merge function allows you to convert the Thru into a secondary input and interlace the incoming stereo signal (Thru + Input). For instance, you could connect the L and R channels of a keyboard and sum them up into the mono output.
Low-Cut / Hi-Pass: a simple low-cut or hi-pass filter
+48V Led: an indicator, present only in Active DI boxes, that informs if the equipment is properly receiving +48V from the mixing console or mic-pre amp.
Ground Lift: sometimes, even with a DI, you will still get a noisy signal. This can be due to several factors: bad cables, bad connectors, a faulty Pre-Amp channel, and the list goes on. But, it can also be due to Ground Loops. The most common symptom of a ground loop is a persistent hum around 50-60Hz, depending on your local utility frequency.
Ground loops occur when you have multiple ground paths between two interconnected pieces of equipment.
For example: When recording keyboards, your instrument is connected to a power outlet, which in turn is connected to the building grounding system. The mixing console is also connected to a power outlet, which is also connected to the building grounding system. When you interconnect with a DI, their ground circuits are no longer isolated from each other, forming a closed-loop, a ground loop.
Ground lift breaks the ground connection between the equipment, eliminating the loop and therefore solving the problem. So before replacing cables and rerouting your signal, try smashing the Ground Lift switch and see what it does for you.
Conclusion
A few points to take away:
- Do not skip on using a DI whenever deemed necessary
- Use the shortest possible Hi-Z cable (instrument cable), avoiding anything over 3m or 10ft in length.
- Use the right tool for the right task:
- For stereo sources, preferably go with a stereo DI box
- For active sources, use passive DI boxes – and vice-versa, but there is some room for experimenting.
